Presumably because the GA, for the calculations not CAT2, assume a GA at DA for normal Cat 1 approach (presumably 200') and you would therefore make the 5% gradient for the GA.
In the CAT 2 case the GA from the CAT 2 DH (of presumably about 100' RA) the aircraft will not make the 5% GA gradient, so a lighter weight is required to make the 5%.